1. Field of Invention
This invention relates to a recorder for effecting waveform recording on a recording paper using a line thermal head; and, more particularly, to improvements thereof resulting in finer stepped portions attributable to the feeding pitch of the recording paper.
2. Description of related art
Known in the art is a recorder for recording waveforms in which recording is effected by a plurality of heating elements, arranged at fixed intervals for constituting a line thermal head, being driven and heated so as to develope color in a thermosensitive recording paper on the basis of the heat thus produced; or by use of ink on an inked ribbon being transferred onto the recording paper.
FIG. 1 is a block diagram depicting a conventional recorder, and FIGS. 2(a),2(b) are timing charts for explaining operation of the recorder of FIG. 1. Data corresponding to m dots for one line, as shown in FIG. 2(a), are consecutively stored in a shift register 1 in response to m clock pulses CLK, shown in FIG.2(b). When data for one line are stored in shift register 1, the data are latched by latches 2, by means of latch pulse LAT, shown in FIG. 2(c). Output data from latches 2 are applied to a respective input terminal of each of a plurality of NAND gates 3. An enable signal EN' shown in FIG. 2(d) is commonly applied to the other input input terminal of each of the NAND gates 3 via an inverter 4. It should be noted that the apostrophe (') indicates that the signal operates on the basis of negative logic. The output terminal of each of NAND gates 3 is connected to one end of each of a plurality of heating element 5 (which constitute a line thermal head). A plus terminal of a DC power source 6 is commonly connected to the other end of each of heating elements 5.
In this circuit, interpolated data are added as recording data for each line in such a manner as to simultaneously drive the plurality of heating elements that are arranged continuously so as to record a maximum value and a minimum value of a measured value in each measurement period, for example, the heating elements include one corresponding to the maximum value and one corresponding to the minimum value.
As a result, as shown in FIG. 2(d), during time t.sub.o, when enable signal EN' is a level L, a driving current flows across the heating element from the DC power source 6, and recording is effected on the basis of the one line of recording data. Then, a recording paper (not shown) is fed one line at a time at a predetermined pitch upon completion of the recording operation of one line.
Disadvantageously, with such conventional arrangement, since the recording paper is fed one line each time the recording of one line is completed, a large stepped portion between the recorded lines appears due to the feeding pitch P of the recording paper, as shown in FIG. 3. It should be noted that arrow Y indicates the feeding direction of the recording paper. This stepped portion becomes large as the feeding speed of the recording paper increases, and results in unfavorable reproducibility of the measured waveform.
If the driving speed of the heating elements 5 is increased, the heating elements 5 are driven before their temperature is lowered sufficiently, so that the temperature becomes gradually higher. Thus, in cases where a thermosensitive recording paper is used, the area which undergoes color development becomes large and the printing quality is lowered. In the worst case, burning of the heating elements may result.
Accordingly, in order to overcome the above deficiencies, thermal history control is effected whereby the amplitude and width of the pulse applied to the heating elements are controlled on the basis of the history of the driving cycles of the heating elements. However, this approach does not use accumulated heat temperature data of the heating elements, so that high accuracy control cannot be expected.